The possibility of utilizing hydrogen as a reliable energy carrier for both stationary and mobile applications has gained renewed interest in recent years due to improvements in low temperature fuel cells and a reduction in hydrogen production costs. There are a variety of ways to store hydrogen, and the more conventional methods include compressed gas (typically at a pressure of 350 or 700 bar) and liquefaction, where the hydrogen is cooled to below its boiling point (20 K). However, these options are costly and require extremely high pressures or low temperatures to achieve reasonable hydrogen densities.
Metastable hydrides (also referred to as metastable hydrogen carrier interchangeably hereinafter) can offer high volumetric and gravimetric hydrogen densities and rapid hydrogen release rates at low temperatures. Unlike other kind of metal hydrides, such as reversible metal hydrides, metastable hydrogen carriers rely on kinetic barriers to limit or prevent the release of hydrogen and therefore can be prepared in a stabilized state far from equilibrium. The rapid, low temperature hydrogen evolution rates that can be achieved with these materials offer much promise for fuel cells such as mobile proton exchange membrane (PEM) fuel cell applications. Applications of the PEM fuel cells may include soldier power, unmanned aerial vehicles (UAV), un-manned underwater vehicles along with fuel cell vehicles.
One of the challenges with a kinetically stabilized hydrogen carrier, such as the metastable hydride, is controlling the release of hydrogen to match the demand from the fuel cell or other energy conversion devices. Left unchecked, the hydrogen pressure can become too high, requiring venting to the environment, or fall too low, starving the fuel cell or energy conversion device.